Certain processes, such as combustion of carbon containing fuels, produce gaseous emissions of CO2. CO2 has been identified as a “greenhouse” gas, which appears to contribute to global warming. Because of its status as a “greenhouse” gas, technologies have been developed to decrease CO2 emissions, including: energy efficiency improvements; increased use of renewable energy sources such as solar and wind; promotion of CO2 absorption/conversion in nature (reforestation); reduction in use of fossil fuels; etc.
Notwithstanding these efforts and technologies, fossil fuels continue to provide a substantial portion of the energy generated today. In addition to developing and exploring energy improvements and alternatives, technologies have also been developed to limit or prevent CO2 release from combustion, including CO2 capture and storage (CCS). Employing CCS technologies may substantially reduce the amount of CO2 released to the atmosphere.
There are various methods or techniques utilized for CCS. Technologies generally fall into one of three areas: post combustion capture (PCC), where CO2 is removed after combustion through absorption or other techniques; pre-combustion (PC), where the fuel is converted before combustion such that CO2 is not a product of energy production; and oxy-firing combustion where the fuel is burned in enriched or pure oxygen instead of ambient air to concentrate the CO2 combustion product.
Drawbacks of CCS technologies include increased capital costs and additional energy consumption associated with the technologies. For example, oxy-firing combustion creates a more concentrated stream of CO2, thereby reducing energy and capital costs of processing a flue stream to capture the CO2 for later storage; however, oxy-firing typically requires an energy demanding air separation unit to generate pure or enriched oxygen. To address this drawback, chemical looping combustion (CLC) has been developed using oxygen carriers to deliver oxygen to a fuel reactor. The oxygen carrier is first oxidized in an air reactor and then oxidized carriers are transmitted to a fuel reactor where they are reduced in contact with the fuel. The carrier is then returned to the air reactor for re-oxidation. By using a carrier, combustion is accomplished in an oxygen rich atmosphere without cryogenically creating oxygen rich gas.
Although oxy-firing and CLC systems generally create a flue stream with a higher concentration of CO2 and substantially reduced concentrations of non-reactive components of air (inerts), such as nitrogen, a flue stream will include H2O as a product of combustion. Moreover, even after initial heat recovery, a flue stream will still have elevated temperatures. CO2 capture and processing is generally accomplished in a gas processing unit (GPU), which separates the CO2 for removal, storage, or reuse, as appropriate. Before a flue stream may be processed in a GPU, H2O, which is gaseous as it exits a fuel reactor in a flue stream, is generally condensed out of the flue stream, leaving substantially concentrated CO2. The flue stream is also cooled before transmission to a GPU. Energy loss can occur both from a failure or inefficient recovery of heat present in a flue stream and from increased energy expenditures in deliberate cooling of a flue stream prior to transmission to a GPU. Moreover, corrosives, such as sulfur dioxide (SO2), may be present in a flue stream and must be cleaned from the flue stream in order to minimize corrosion risks prior to transmission to the GPU.
A condenser may be used to condense water from a flue stream and cool the flue stream. A condenser may be of various types, including a direct contact condenser, which may be a column condenser having a packed bed element. In a condenser having a packed bed element, external heat exchangers may be incorporated for recovery of heat from the flue gases and to assist in cooling of flue gases; however, there is a requirement that difference between temperatures of the flue gas and temperatures of return condensate should be minimized, typically, not more than 4° C. Generally, higher liquid loads are preferred for maximizing cooling of flue gases. The lower the liquid load employed the more area of packing bed needed to cool the flue gases. Increasing the liquid load, however, will cause the temperature difference between flue gases and return condensate to exceed the temperature difference requirement. In order to minimize the difference in temperatures, the liquid load through the packed bed element is reduced, which lowers efficiency of heat transfer from flue stream to condensate flow and reduces cooling of the flue stream. Because of the requirement to minimize the temperature difference, requiring a low liquid load through a packed bed element, a high tower is required to sufficiently condense H2O, which sacrifices cooling of the flue gas. Accordingly, there is a need for an improved condenser and method of cooling a flue stream to more efficiently recover heat from flue gases, cool flue gases for transmission to a GPU and remove sulfur dioxide from the flue gases to avoid corrosion risks.